1. Field of the Invention
The present invention relates to an adaptive optics apparatus that corrects aberration of an examination object, and an image taking apparatus including an adaptive optics apparatus, and more specifically, it relates to a technique suitable for an optical image taking apparatus used, for example, for ophthalmic care and including an adaptive optics that corrects aberration of an eye.
2. Description of the Related Art
In an apparatus that is used, for example, for astronomical observation or fundus examination and measures an object using light, factors that lie between a measurement object and a detection optical system and fluctuate with time or vary greatly from individual to individual affect the imaging performance of the optical system and in some cases make it impossible to obtain sufficient resolution. These factors are caused, for example, by atmospheric fluctuation, change in a tear film of an eye, or individual difference in imperfection of an ocular optical system, include high-order optical aberration components, and are often not sufficiently corrected by an optical system including lenses and mirrors. A technique of an adaptive optics (hereinafter referred to as AO) is known that measures the changing aberration and feedback-controls the aberration using a device capable of dealing with high-order aberration.
In the case of fundus examination (in this case, the examination object is an eye), it is desirable to identify a photoreceptor cell unit on a retina, the size of which is of the order of magnitude of 2 to 3 μm. Ultimately, it is desirable to achieve an optical biopsy capable of cellular imaging in the near future. In order to achieve this, a spatial resolution of 1 μm or less comparable to a microscopical image of a piece of tissue is required. The wavelength of light used for such retina examination must be within the near infrared or infrared region in which light is poorly absorbed by water, which is the major component of the body tissue, and is poorly scattered in the tissue. In order to obtain a lateral resolution of 1 to 3 μm, the diameter of a beam incident on the pupil needs to be about 6 to 8 mm. Actually, in an optical system of an eye, from a cornea to a vitreous body, the state of the curved surface and the refractive index of each tissue are often not uniform. Even when an eye is observed with a beam having a diameter of about 6 to 8 mm, the wavefront of light is distorted in the eye, and a desired resolution cannot be obtained.
In Opt. Express 13, 8532 (2005) is reported a method to obtain desired light-collecting performance by using the AO technique, detecting aberration of a wavefront distorted in an eye, and compensating for the aberration with an aberration corrector. The AO in Opt. Express 13, 8532 (2005) employs a deformable mirror (hereinafter referred to as DM) including a wavefront aberration detector and 35 actuators. It is reported that an optical coherence tomography (hereinafter referred to as OCT) having such an adaptive optics achieved a resolution in the lateral direction of 4 μm and a resolution in the depth direction of 6 μm. In the case of such a DM, the optimal mirror shape to correct the aberration is formed by pushing and pulling a continuous surface mirror with several tens of actuators. Therefore, interaction between actuators occurs and complicates control calculation. Depending on the aberration shape, a desired shape cannot be sufficiently reproduced.
Japanese Patent Laid-Open No. 2007-014569 proposes an ophthalmic photographing apparatus including an AO having a wavefront aberration corrector that is a spatial light modulator (hereinafter referred to as SLM), which has superiority over a DM in wavefront reproducibility. The apparatus of Japanese Patent Laid-Open No. 2007-014569 is a scanning laser ophthalmoscope (hereinafter referred to as SLO) that scans a laser beam to obtain an fundus image. The SLO includes an AO that employs an SLM that uses orientational control of liquid crystal. Optical distance is the product of refractive index n and geometrical distance d. The above-described DM corrects a wavefront by changing d. In contrast, an SLM can correct wavefront aberration by changing n and thereby changing a wavefront. In the case, for example, of an SLM using liquid crystal, the number of pixels is very large, there is little interaction between pixels, and therefore control can be performed independently. Consequently, the SLM has superiority over the DM in wavefront reproducibility.
However, an AO that employs an SLM that uses orientational control of liquid crystal, such as that in Japanese Patent Laid-Open No. 2007-014569, has the following problems. When an SLM that uses orientational control of liquid crystal is used as in Japanese Patent Laid-Open No. 2007-014569, only light in a particular polarization direction is modulated. Therefore, when linearly polarized light (intrinsic polarized light) polarized in that direction is incident, ideally, 100% of incident light can be modulated. Therefore, when linearly-polarized laser or light from an SLD (Super Luminescent Diode) light source is incident on the SLM, high modulation efficiency can be achieved.
When such light is incident on the SLM, a polarized light component perpendicular to the modulation direction cannot be modulated, and outgoing light from the SLM includes two polarized light components that differ in spatial phase state (differ in wavefront). The component that is not the intrinsic polarized light of the SLM is not efficiently modulated, the wavefront cannot be sufficiently corrected, and therefore a desired resolution cannot be obtained. Disposing a polarizer that is parallel to the modulated polarization direction in front of the SLM makes it possible to select modulated light component. However, the polarized light component that is not modulated is removed, and therefore light use efficiency is significantly lowered. A retina has a low reflectivity, and the power of light illuminating it is limited by standards for safety. In the case of an examination object like a retina, it may be difficult to obtain a good-quality image when light quantity loss is about half.